Lithium ion nonaqueous electrolyte batteries have widely proliferated as batteries having a high energy density in various fields, including electric automobiles, power storage, and information devices. Due to this, demands on such batteries have increased, and studies thereof have gathered pace.
Especially, it is required for lithium ion nonaqueous electrolyte batteries used for a power source for electric automobiles to have a high energy density, i.e., to have a large discharge capacity per unit mass or unit volume, in view of their use. In addition, it is required that in order to regenerate kinetic energy during braking, efficient charge can be performed even if a large current is instantly input into a battery. Furthermore, it is required, contrary to the above, that a large output can be performed, i.e., a large current can be instantly discharged at the time of starting, sudden starting, or sudden acceleration. It is required, accordingly, for the secondary battery for the power source for the electric automobile, to have good input/output characteristics for a short time, in addition to a large capacity.
Carbon materials have been frequently used as a negative electrode active material in the lithium ion nonaqueous electrolyte battery. Recently, however, a spinel type lithium titanate having a higher Li insertion/extraction potential than that of the carbon material has received attention. The spinel type lithium titanate has excellent cycle characteristics because it has no volume change caused by a charge-and-discharge reaction. In addition, the spinel type lithium titanate has a high safety, because a possibility of generation of lithium dendrite is lower compared to a case where the carbon material is used, and has a great merit in which it is difficult to cause thermal runaway because it is a ceramic.
On the other hand, a nonaqueous electrolyte battery using the spinel type lithium titanate as a negative electrode active material has a defect of a low energy density, and thus negative electrode materials capable of obtaining a high capacity are required. Therefore, niobium-titanium composite oxides such as Nb2TiO7, which has a geometric capacity per mass larger than that of a spinel-type lithium titanate, Li4Ti5O12, have been studied.